Beams of ions have found increasing use in the fabrication and diagnosis of many types of devices, particularly in the field of microelectronic devices and circuits. Some applications for ion beams are described in ION BEAMS by R. G. Wilson and G. R. Brewer, published by John Wiley and Son, 1973 in Chapter 4, pages 261-398. An important part of an ion beam system is the ion source. Wilson et al. in Chapter 2, pages 11-128, describe various types of ion beam sources. One type of ion beam source commonly used is a hot filament cathode source described on pages 48-58 in Wilson et al., and more particularly, a plasmatron source described on pages 61-66. A plasmatron, in general, includes a filament type cathode, an intermediate electrode in front of the cathode and an anode in front of the intermediate electrode. The intermediate electrode and the anode have aligned openings therethrough which allow the emission of the ions. A gas to be ionized is emitted into the system, generally between the anode and the intermediate electrode and flows to the filament cathode.
In the operation of the plasmatron, the filament cathode is heated and an arc discharge, a plasma, is created between the heated cathode and the anode. This generates electrons which are accelerated from the cathode to the aode through the gas to be ionized. The plasma is constricted by the intermediate electrode which is at a potential intermediate the cathode and anode. This creates a plasma bubble bounded by a charge double layer that focuses the electron from the plasma on the cathode side to the region in front of the ion extraction aperture. In a unoplasmatron, the ions are extracted from the bubble. A duoplasmatron includes an electromagnet around the plasma which creates a magnetic field. The magnetic field confines the electrons so as to limit intense ionization to a small region around the anode aperture. Extraction electrodes are generally provided in front of the anode to extract the ions.
This type of ion beam source has been found to have several problems. A major problem is that the hot filament cathode does not have a very long life when operating with helium as the discharge gas, generally only about 50 hours. Another problem which has arisen is the ability to start the operation of the device.